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ENCYCLOPEDIA ARTICLE

Food microbiology

A subdiscipline in the field of microbiology concerned with the study of bacteria, fungi, and viruses thatgrow in or are transmitted by foods. While bacteria are frequently associated with food spoilage and foodpoisoning, some species preserve foods through fermentation or produce food ingredients. Foodmicrobiology is a broad field that can include not only microbiology but also sanitation, epidemiology,biochemistry, engineering, statistics, and mathematical modeling.Pathogens and spoilage organisms

Some people dismiss food poisoning as a minor annoyance. In reality, the suffering and economic lossesstemming from food-borne pathogens are substantial, but they are often hidden. Annual economic lossesfrom food-borne pathogens are extremely high. Salmonella, which cause an average of 40,000 casesyearly and 2000–3000 deaths in the United States, are responsible for about a third of these losses.Individual outbreaks of food-borne diseases can affect thousands of people. Many outbreaks arepredictable and preventable through good sanitation, preservatives, thermal processing, and refrigeration.More than half, however, are of unknown etiology are poorly understood, and may be caused by so-callednew pathogens. See also: Food poisoningHistorical pathogens

In the 1960s, most food-related illnesses were attributed to one of five major groups of pathogenicbacteria. These were associated with particular foods, commodities, or processes and were classified asinfectious or toxin-producing. These five groups, described below, remain major causes of food-borneillness. See also: BacteriaSalmonella and Shigella

The primary infectious bacterium associated with foods is Salmonella. These organisms causegastroenteritis with symptoms of fever, diarrhea, and vomiting 12–36 h after ingestion. Salmonellosis isusually self-limiting, but it can be fatal in the old, young, or medically compromised individuals. Salmonellaare commonly found on meats, especially poultry and eggs. Salmonella are easily killed by cooking.However, items contacted by the contaminated raw meat can transfer the Salmonella to food that is readyto eat (cross contamination) and cause illness. The seasonal increase in Salmonella isolations illustrateshow food-borne illness increases in warm summer months.Shigella are related organisms which produce a similar infectious syndrome. They are usually transmittedby a fecal-oral route or through feces-contaminated water rather than through foods.Clostridium botulinum

The most dreaded toxin-producing organism is Clostridium botulinum. It excretes a potent neurotoxin thatcauses weakness, double vision, slurred speech, paralysis, and often death if ingested. The vegetativereproductive form of C. botulinum is heat-sensitive, lives only in the absence of air, does not compete wellwith other bacteria, and is rarely a problem in fresh foods. Clostridium botulinum spores are killed onlythrough severe heating, such as in canning.Page1of6McGraw-Hill's AccessScience3/23/2010http://www.accessscience.com/popup.aspx?id=267000&name=printHistorically, botulism has been associated with foods canned at home. If canned foods receive inadequateheat processing, competing bacteria are killed, air is expelled, and the botulinal spores germinate.Fortunately, botulinal toxin is often destroyed by heat when the food is cooked before serving; hence thestandard advice is to boil home-canned foods before eating.Modern commercial canning is designed to destroy C. botulinum spores. Reported outbreaks of botulismcaused by pot pies, potatoes, and fried onions have been caused by temperature abuse, that is, theholding of foods at warm temperatures that promote bacterial growth. Clostridium botulinum can also be aproblem in processed meats, such as hams and sausages. In this case, its growth is controlled through theuse of nitrite, salt, and refrigeration. One type of C. botulinum is associated with fish. See also: Botulism;ToxinClostridium perfringens and Bacillus cereus

These are spore-forming, toxin-producing bacteria that cause illness when foods are heated enough to killcompeting bacteria but not enough to kill the spores. When large volumes of foods are prepared, cooked,and then kept warm until they are served, spores can germinate. In the case of C. perfringens, which isassociated with meats, the ingested cells release toxin in the digestive tract, resulting in cramps anddiarrhea. Bacillus cereus, found in meats, dried foods, and rice, produces two different types of toxins: thediarrheal toxin, which has an etiology similar to C. perfringens, and the emetic (vomiting) toxin, whichcauses symptoms similar to those produced by staphylococcal toxins.Staphylococcus aureus

This bacterium produces toxins that are very resistant to heat. Staphylococcus aureus is found in the noseand throat of many healthy people and is transferred to food by inadequate hygiene. When foods aretemperature-abused, the bacteria grow and produce toxin. Subsequent heating of the food kills thebacteria but does not inactivate the toxin. The toxin causes severe vomiting and diarrhea from ½ to 4 hafter ingestion. The microorganism grows well at salt and sugar concentrations that inhibit manycompeting bacteria. Foods high in protein, such as cured meats, custards, and cream-filled bakery goods,pose special hazards for staphylococcal food poisoning. See also: StaphylococcusMicrobial ecology of foods

Modern food microbiology views foods as habitats where different organisms compete for survival. The factthat there are 250 genera of bacteria and that only 25 of these (8 pathogenic) are found in foods suggeststhat foods provide unique ecological niches. Viruses do not reproduce in foods and are not competitors inthis sense (the food acts only as a carrier). Yeasts and molds usually grow more slowly than bacteria andare rarely a problem in foods that support bacterial growth. See also: Fungi; YeastBacteria reproduce by binary fission; it takes only 20 doublings for one cell to yield more than 1 millioncells. In environments where the doubling time is short, this occurs quite rapidly. Many preservationmethods alter foods' environmental conditions in order to slow microbial growth. See also: BacterialgrowthTemperature

The most important environmental condition is temperature. Most food-borne pathogens are mesophiles;that is, body temperature is optimal for growth. With a doubling time of 20 min at 98.6°F (37°C), onebacterium generates 1 million progeny in less than 7 h; at 32°F (0°C) the doubling time increases to 1200min and the 1 million cell count is not reached for 16 days. Keeping hot foods hot (>145°F or 63°C) andcold foods cold (<45°F or 7°C), combined with rapid heating and cooling to get rapidly beyond the growth-Page2of6McGraw-Hill's AccessScience3/23/2010http://www.accessscience.com/popup.aspx?id=267000&name=printpromoting temperature range (45–145°F or 7–63°C), prevents most food-borne illnesses. Psychrophylic(cold-loving) bacteria such as Listeria monocytogenes are exceptions.Acidity

A food's acidity, quantified as pH, is another major environmental factor. The pH range for bacterialgrowth is 4–9, with fastest growth at neutrality (pH 7). Changing a food's acidity can change the rate ofbacterial growth. Meats, fish, poultry, and most dairy products are near pH 7, which is ideal for bacterialgrowth; fermented foods and fruits have pH less than 4. Many yeasts and molds grow in acidicenvironments and spoil acidic foods. The pH value of 4.6 has special significance because C. botulinum cangrow and produce toxin above this value. Canned foods with pH above 4.6 are legally classified as low-acidand must be processed in retorts under steam at 240–280°F (116–138°C) to kill C. botulinum spores.Foods with pH below 4.6 are legally high-acid and are processed in open pans of boiling water. In thiscase, C. botulinum need not be killed because it cannot grow at low pH. See also: pHWater activity

The amount of water available for microbial growth, that is, water activity (aw), is the third major factorinfluencing microbial competition. Water activity is the equilibrium relative humidity generated by a food ina closed chamber divided by 100 to give a 0 to 1.00 scale. Salad dressings and honey, which both contain50% water, are microbiologically quite different. The dressing separates into a 100% free-water phase(aw= 1) and supports bacterial growth, while the sugar in honey binds water so tightly that it isunavailable for microbial growth. Most bacteria grow only at aw= 0.90–1.00. Fresh meats, vegetables,fruits, and perishable foods have water activity in this range. Most yeasts can grow at slightly lowervalues. Staphylococcus aureus is the pathogen most insensitive to water; it grows at aw= 0.86. Since nopathogenic bacteria grow below aw= 0.85, this value has special significance in the regulations defininglow-acid foods. Foods having an awvalue below 0.85 are legally considered high-acid, regardless of theirpH. Most molds grow at awvalues as low as 0.8 and compete well in foods such as flour, cakes, beans,rice, and cereals. Some xerophilic molds and yeasts grow at awvalues as low as 0.6. Dehydrated foods,with even less available water, are completely recalcitrant to microbial spoilage.Oxygen

Oxygen can be favorable, neutral, or inhibitory to bacterial growth. In one process, foods are first vacuum-packed to inhibit aerobic spoilage organisms and are then partially cooked. This environment is perfect foranaerobic spore-forming microorganisms. However, the Food and Drug Administration prohibits the use ofthis process because of the potential botulinal hazard.Preservatives

Chemical preservatives also render food environments unsuitable for microbial growth. The oldestpreservative is common table salt; at very high levels, it produces water activities that are inhibitory tomicrobial growth, although many organisms are inhibited by as little as 3% salt. Nitrites are used in curedmeat as anticlostridial agents. Acetic, lactic, citric, benzoic, and propionic acids and sodium diacetate canalso be added to foods as microbial inhibitors. Considering the low levels used, the long history of safeuse, and the consequences of microbial growth, the risk:benefit ratio associated with chemicalpreservatives is very low.Multiple barriers

Consumer preferences for fresh and natural foods make it difficult to alter any one environmental factorenough to inhibit microbial growth, and so the trend is to use multiple barriers, or hurdles. This approachPage3of6McGraw-Hill's AccessScience3/23/2010http://www.accessscience.com/popup.aspx?id=267000&name=printemploys several inhibitors at suboptimal levels. For example, clostridia may be inhibited by 7% salt at pH7.0 or 0% salt at pH 4.6, but a meat treated this way would be unacceptable, tasting either salty or acidic.However, 3% salt at pH 6 in the presence of nitrite at a concentration of 125 parts per million providesmultiple barriers sufficient to inhibit the bacteria and not impair flavor.Emerging pathogens

The demand for longer shelf life in refrigerated foods combined with their increased popularity has causedrenewed concern about psychrophilic pathogens, such as Yersinia enterocolitica, and enterotoxigenicorganisms, such as Escherichia coli and Listeria monocytogens. These bacteria grow most rapidly at 59–86°F (15–30°C) and, at refrigerated temperatures, can succesfully compete with the normal mesophilicbacteria, thus limiting the shelf life of refrigerated foods. Listeria cause special concern because they infectwomen and their unborn children preferentially. See also: Escherichia; Listeriosis; YersiniaCampylobacter jejuni is a pathogen that is responsible for more illnesses than Salmonella and Shigellacombined. Ingestion of relatively small numbers can cause diarrhea, cramps, and nausea. This organism ismicroaerophilic (requires 5–10% oxygen), is relatively fragile, and shows a seasonal pattern of outbreakssimilar to Salmonella. It is associated with raw meats and unpasteurized milk, and can be controlled bypasteurization, heating, and good sanitation.Analytic approaches

Microbial analysis of foods frequently requires “zero defects” in the absence of 100% testing. Legally,ready-to-eat foods must be free of Salmonella. This demands that the food microbiologist be able to detectone Salmonella among millions of innocuous bacteria in a pound of food. Moreover, all of the food cannotbe tested because microbial analysis is destructive. Therefore, statistical sampling plans determine howmany samples must be tested to have confidence that the whole lot is free of Salmonella.In the classical methods for counting microorganisms, a food or its hemogenate is highly diluted so thatonly 30–300 cells are transferred to growth media. After 2–10 days, each cell grows into a colony, andthese are counted and multiplied by the dilution factor to estimate the number of cells in the food.Automated methods have been developed that measure growth products, bacterial deoxyribonucleic acid(DNA), or specific toxins; these methods dramatically reduce the analysis time and are rapidly replacingthe petri-dish method.A procedure known as hazard analysis critical control points (HACCP) can replace much postproductiontesting. This technique examines a food, its ingredients, and its processing to identify points critical tosafety. These points are then heavily monitored during production; if they are maintained, a safe productresults.Beneficial food-borne organism

When certain bacteria grow in foods, they produce desirable flavors and textures, and may also inhibitpathogenic organisms. Most of these bacteria belong to the genera Streptococcus, Lactobacillus,Leuconostoc, Pediococcus, or Micrococcus. They are used to make fermented dairy products, meats, andvegetables, and to preserve food by converting the sugars needed by competing microbes to lactic acid,which inhibits their growth. These lactic acid bacteria are unusually tolerant of acidic environments.Acetobacter and Gluconobacter are used in the production of vinegar. Yeasts, usually Saccharomyces,which produce ethanol and carbon dioxide, are used in the processes of brewing and baking.Lactid acid coagulates casein, the major protein in milk, and this process is used to manufacture cheese.During the aging of cheese, bacterial enzymes generate characteristic flavors, allowing a wide variety ofPage4of6McGraw-Hill's AccessScience3/23/2010http://www.accessscience.com/popup.aspx?id=267000&name=printproducts to be made by using many different bacteria. The bacteria used can be indigenous to the milk,derived from a previous fermentation, or added as pure cultures. See also: Cheese; FermentationUntil the late 1960s, staphylococcal food poisoning was a major problem in certain meat products, such asbologna, pepperoni, and salami. Since acid production by indigenous bacteria is often unpredictable, it isnow recommended that defined starter cultures be used to ensure that sufficient acid is produced earlyenough to prevent staphylococcal growth.A novel use for starter cultures of lactic acid bacteria is to prevent botulinal growth in bacon. A smallamount of culture and sugar are added to the cured meat; if the bacon is temperature-abused, the lacticacid bacteria grow and produce acid to inhibit botulinal growth.Certain vegetables are preserved by fermentation. Pickles are made by fermenting cucumbers; olives andmany oriental foods are also fermented. During sauerkraut fermentation, the addition of 2.5% salt (byweight) to shredded cabbage selects for the growth of Leuconostoc mesenteroides, which stop growingwhen acid levels reach about 0.067%. This environment favors Lactobacillus plantarum, which producesacid to levels of 1.25%, which is tolerated by L. brevis, and this bacterium brings the product to a finalacidity of 1.7%.Biotechnology

Advances in molecular biology have generated interest in applications to food processing.The most important contribution of biotechnology to food microbiology is the production of probes thatdetect pathogenic organisms much faster than conventional methods. For example, conventional methodsrequire 5 days to confirm the presence of Salmonella in foods; probes that detect Salmonella-specific DNAor antigens can give similar results in 2 days.The dairy industry has benefitted from advances in biotechnology by acquiring the ability to determine thegenetic basis for the bacterial metabolism of lactose in milk and to stabilize it. In addition, enzymes thataccelerate the aging of cheese have become commercially available, making it possible to produce acheese with the taste of 9-month-old cheddar in just 3 months. See also: Biotechnology; Enzyme; Foodengineering; Food manufacturing; Food preservation; Genetic engineering; VirusThomas J. MontvilleBibliography